Field of The Invention
The invention relates generally to the field of turbomachines, and more particularly that of turbine blades of these turbomachines and to their manufacture.
Description of the Related Art
Turbine blades are subjected to strong thermal stresses due to the heat in gases in which they are plunged at the outlet of the combustion chamber, and need to be cooled to support these temperatures. They are accordingly hollow and traversed by internal cavities in which cooling gas circulates, taken at the outlet of a stage of one of the compressors.
More precisely, a turbine blade of a turbomachine comprises an aerodynamic surface (or blade) extending between a blade foot and a blade tip. The blade has a leading edge arranged opposite the flow of hot gases coming from the combustion chamber of the turbomachine, as well as a trailing edge opposite the leading edge and the lateral intrados and extrados walls which connect the leading edge to the trailing edge.
The internal cavities extend over the height of the blade, and comprise, from upstream to downstream in the direction of the flow of gases from the combustion chamber, a leading edge cavity and a trailing edge cavity, adjacent to the leading edge and the trailing edge of the blade respectively, and at least one central cavity, extending between the leading edge cavity and the trailing edge cavity. These cavities are fed with cooling gas via tubing connecting them to the foot of the blade.
The blade also comprises, at the level of its tip, a hollow form or bath, which is defined by the extension of the intrados and extrados walls, as well as by a bottom wall which close off the internal cavities.
To make these different cavities, which have complex forms and whereof the geometry must be respected with great precision, the blades are classically produced by a technique known under the name of lost wax smelting. This technique consists schematically of making a blade draft of wax in which cores made of ceramic which reproduce the resulting cavities are embedded. The wax blade is then embedded in a carapace, for example made of refractory material, then the cores are eliminated chemically, leaving in their place the preferred internal cavities and bath. Embodiments of this method are described in particular in documents FR 2 875 425, FR 2 874 186, or FR 2 957 828 in the name of the applicant.
The cores for these modern turbine blades are constituted by internal cavity cores, having classically the form of columns, which are positioned side by side and held together by conventional means.
These cores have increasingly more complex forms, as the specifications required for cooling of blades grow and the blades diversify. It is necessary to position them in the carapace with extreme precision.
A core generally comprises a first core element designed to form the cavities and a second core element designed to form the bath, the second core element being connected to the first core element by linking rods made of alumina or quartz.
The aim of these rods is to hold between them the parts of the core and stiffen the resulting assembly, and they are involved in making dedusting holes in the upper part of the blade. These rods are stored in the holes which they tend to make in the bottom of the bath. The dedusting holes enable circulation of the cooling gas in the cavities and evacuation of various particles entering the turbomachine.
To improve the aerodynamic performance of the blade and minimise energy losses, it has been proposed to use turbine blades for turbomachines having an advanced blade tip of the type “offset of tip sections” according to the French patent application registered on Nov, 17 2011 No. FR 11 60465 in the name of the applicant.
Such turbine blades are adapted to minimise energy losses. They comprise a blade which can be broken down into blade sections stacked according to a stacking direction along the blade. In the case of the blade tip with tip section offset, the stacking of the sections at the level of the tip of the blade is offset in the direction of the intrados wall, preferably progressively.
For this, as described in patent application No. FR 11 60465, the blade can comprise a cavity at the level of its tip, open in the direction of its free end and delimited by the bottom wall and a rim which extends between the leading edge and the trailing edge. The stacking of the blade sections of the blade at the level of this rim presents offset in the direction of the intrados, this offset increasing as the free end of the tip of the blade is approached. The blade also comprises cooling channels, inclined relative to the intrados, and connecting the internal cavities to the intrados wall.
The intrados wall of the blade can also present a projecting portion, whereof the outer face is inclined relative to the rest of the intrados of the blade and has at its end a terminal face, turned towards the rim. The bottom wall is connected to the intrados wall at the level of the terminal face of the projecting portion, and the cooling channels can be arranged in the projecting portion of the intrados wall such that they terminate on the terminal face of the projecting portion, the distance between the axis of the cooling channels and the outer limit of the free end of the rim of the intrados side being greater than zero.
However, this tip section offset and the small size of the blade, and therefore of the cores used for its manufacture, make it difficult to hold the rods of the second core element which is designed to former the bath on the first core element.
It has therefore been proposed to orient the rods individually, with big angles relative to the main direction of the blade. However, the cores are complex to produce due to the strong inclination of the linking rods relative to the main direction of the cores (and therefore of the injection of ceramic), which can raise problems of wear of the cores at the level of the bottom of the bath. Also, executing this manufacturing method needs knowhow and experience which are accessible to all those skilled in the art, specifically the founders, as here.
It has also been proposed to use linking rods anchored conventionally in the core elements, but with anchoring strongly reduced in comparison with conventional techniques, due to the minimal dimension of the internal cavities of the blade. But, the anchoring depth and the thickness of the cores (generally made of ceramic) about the linking rods cause problems of cracking (partial ruptures which occur under the action of forces resulting from uneven withdrawal) in the core elements, and therefore an excessive rate of discard.